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January 11, 2012

About one out of every ten stars has an earth size planet in the habitable zone per a gravitational microlensing survey


Approximately one out of every ten stars have a planet roughly the size of the Earth with an orbit that, if there was water and atmosphere, would create a temperature and climate roughly that same as on Earth – we could live there

University of Copenhagen - Six years of observations of millions of stars now show how common it is for stars to have planets in orbits around them. Using the gravitational microlensing method that is highly sensitive to planets that lie in a habitable zone around the host stars, astronomers, including members from the Niels Bohr Institute, have discovered that most of the Milky Way’s 100 billion stars have planets that are very similar to the Earth like planets in our own solar system – Mercury, Venus, Earth and Mars, while planets like Jupiter and Saturn are more rare. The results are published in the prestigious scientific journal, Nature.

“Our results show that planets orbiting around stars are more the rule than the exception. In a typical solar system approximately four planets have their orbits in the terrestrial zone, which is the distance from the star where you can find solid planets. On average, there are 1.6 planets in the area around the stars that corresponds to the area between Venus and Saturn” explains astronomer Uffe Gråe Jørgensen, head of the research group Astrophysics and Planetary Science at the Niels Bohr Institute at the University of Copenhagen.


Gravitational microlensing method requires that you have two stars that lie on a straight line in relation to us here on Earth. Then the light from the background star is amplified by the gravity of the foreground star, which thus acts as a magnifying glass. When astronomers observe the light from the background star there might be a little extra bump on the light curve if there is a planet around the foreground star.

Nature - One or more bound planets per Milky Way star from microlensing observations




Habitable exoplanets

In order to find planets similar to the planets we know from our own solar system, researchers must use a third method – gravitational microlensing observations. But the gravitational microlensing method requires very special conditions concerning the stars location in the galaxy.

Uffe Gråe Jørgensen explains that you need to have two stars that lie on a straight line in relation to us here on Earth. Then the light from the background star is amplified by the gravity of the foreground star, which thus acts as a magnifying glass. When the stars pass close by each other in the sky, astronomers can observe the light from the background star first increase and then decrease again. If there is a planet around the foreground star, there might be a little extra bump on the light curve. But if the planet is very close to the star, the bump ‘drowns’ on the light curve, and if the planet is very far from star, you do not see it. “Therefore the method is most sensitive to planets that lie at an Earth-like distance from a star,” explains Uffe Gråe Jørgensen.

It is rare that two planets pass by each other closely enough to create a microlens. We have therefore implemented a strategic search on two levels. Every starry night the research group scans 100 million stars using telescopes in Chile and New Zealand. If the scanning identifies a stellar location with a possible microlensing effect, it is automatically registered and all researchers are notified. Then the best ‘lenses’ are observed more closely at high resolution and their light curves are analysed. One of the places this is done is at the Danish 1.5 meter telescope at ESO’s La Silla Observatory in Chile.

“In a six year period from 2002 to 2007, we observed 500 stars at high resolution. In 10 of the stars we directly see the lens effect of a planet, and for the others we could use statistical arguments to determine how many planets the stars had on average. To be exact, we found that the zone that corresponds to the area between Venus and Saturn in our solar system had and average of 1.6 planets the size of five Earth masses or more,” explains Uffe Gråe Jørgensen.

The microlensing results complement the best existing transit and radial velocity measurements. Using transit measurements, the American Kepler satellite has identified a very large number of relatively small planets in orbits smaller than even the innermost planet in our own solar system, Mercury, while many years of radial velocity measurements have revealed a large number of very large planets in both very small orbits and slightly larger orbits.

“Our microlensing data complements the other two methods by identifying small and large planets in the area midway between the transit and radial velocity measurements. Together, the three methods are, for the first time, able to say something about how common our own solar system is, as well as how many stars appear to have Earth-size planets in the orbital area where liquid what could, in principle, exist as lakes, rivers and oceans – that is to say, where life as we know it from Earth could exist in principle,” says Uffe Gråe Jørgensen.
He explains that a statistical analysis of all three methods combined shows that out of the Milky Way’s 100 billion stars, there are about 10 billion stars with planets in the habitable zone. This means that there may be billions of habitable planets in the Milky Way. For thousands of years people have been guessing how many planets there might be out there among the stars, where we could, in principle at least, live. Today we know this.

22 pages of supplemental information

Most known extrasolar planets (exoplanets) have been discovered using the radial velocity or transit methods. Both are biased towards planets that are relatively close to their parent stars, and studies find that around 17–30% of solar-like stars host a planet. Gravitational microlensing on the other hand, probes planets that are further away from their stars. Recently, a population of planets that are unbound or very far from their stars was discovered by microlensing. These planets are at least as numerous as the stars in the Milky Way. Here we report a statistical analysis of microlensing data (gathered in 2002–07) that reveals the fraction of bound planets 0.5–10 au (Sun–Earth distance) from their stars. We find that of stars host Jupiter-mass planets (0.3–10 MJ, where MJ = 318 Mcircle plus and Mcircle plus is Earth’s mass). Cool Neptunes (10–30 Mcircle plus) and super-Earths (5–10 Mcircle plus) are even more common: their respective abundances per star are and . We conclude that stars are orbited by planets as a rule, rather than the exception.

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